The advantages of free-space optical communication (FSO), such as high bandwidth and high signal directivity, have long been known. One of the known drawbacks of FSO is its susceptibility to scintillation. That is, turbulence and thermal phenomena create localized fluctuations in the atmospheric refractive index. A signal-carrying FSO beam passing through such fluctuations may be wholly or partially deflected due to optical refraction. One consequence is that at the receiving end, where the beam is focused onto an image spot, the image spot may wander in the image plane.
Moreover, FSO beams are typically generated as coherent laser beams. A coherent beam initially has a uniform wavefront, i.e., a phase that varies slowly with radial position over the leading face of the beam. A beam having a uniform wavefront is desirable because at the detector, it will experience relatively little destructive self-interference. However, a second effect of index fluctuations in the atmosphere is to distort the wavefront of the FSO beam. The distortions cause time-dependent, destructive self-interference of the beam at the detector, which leads, in turn, to fading of the received signal.
Some of the known solutions to the problems related to atmospheric scintillation use mechanical mirrors and adaptive optics to track the wandering beam and to correct the distorted wavefront. However, such solutions require bulky hardware components. Some of them also use a closed, iterative control loop of sensing and mechanical actuation that limits their performance. These drawbacks are particularly severe in the case of QAM and other advanced modulation methods for the FSO beam, for which reception is relatively sensitive to the fidelity of the amplitude and phase of the received signal.